As Web sites are now ordinary products, it is necessary to explicit the notion of quality of a Web site. The quality of a site may be linked to the easiness of accessibility and also to other criteria such as the fact that the site is up to date and coherent. This last quality is difficult to insure because sites may be updated very frequently, may have many authors, may be partially generated and in this context proof-reading is very difficult. The same piece of information may be found in different occurrences, but also in data or meta-data, leading to the need for consistency checking. In this paper we make a parallel between programs and Web sites. We present some examples of semantic constraints that one would like to specify (constraints between the meaning of categories and sub-categories in a thematic directory, consistency between the organization chart and the rest of the site in an academic site). We present quickly the Natural Semantics, a way to specify the semantics of programming languages that inspires our works. Then we propose a specification language for semantic constraints in Web sites that, in conjunction with the well known ``make'' program, permits to generate some site verification tools by compiling the specification into Prolog code. We apply our method to a large XML document which is the scientific part of our institute activity report, tracking errors or inconsistencies and also constructing some indicators that can be used by the management of the institute.